In the formation of wafer-level chip scale package (WLCSP) structures, integrated circuit devices, such as transistors, are first formed at the surface of a semiconductor substrate in a wafer. Interconnect structures are then formed over the integrated circuit devices and solder balls are mounted on the surface of the wafer. Afterwards, the wafer is sawed into dies.
The dies may be bonded to printed circuit boards (PCBs) directly. The dies that were directly bonded to PCBs were typically small dies. Accordingly, the stresses applied on the solder balls that bond the dies to the respective PCBs were relatively small. Recently, increasingly larger dies are bonded to PCBs. The stresses applied on the solder balls thus become increasingly greater. Consequently, mechanisms for reducing the stresses are needed. Underfill is a mechanism for reducing stress on solder balls. However, underfill is not suitable for WLCSP in protecting the solder balls. If the underfill is applied, the bonding between the dies and the PCBs is not re-workable. Dies with underfill cannot be removed from the respective PCB in the situations that the dies are defective. As a result, the entire PCBs bonded to the failed dies become defective.